Legal Question in Criminal Law in California

You question is quite unclear.

Your statement that "California law requires a trial judge to impanel plaintiffs on the jury" is not a correct statement of the law. Quite the opposite.

A 'plaintiff' who is by definition a Party to the case, obviously can NOT be on the jury on that case.

If you mean that someone related to a party, or otherwise 'secretly' involved in the case somehow, is allowed to become a jury, over the objection of one of the attorneys, that would be grounds fora Motion, mistrial, appeal, or other relief if brought in a timely manner.

If a prospective juror lies or withholds information about his 'bias' or connection to the case, he would be removed if discovered before the trial ends, possibly punished for his misconduct.